Iran has ramped up efforts to target US personnel, and lethal plots by Tehran against American citizens have reached "advanced stages", a senior intelligence official in the Trump administration warned in a memo seen by Reuters.
The alert from Director of National Intelligence Jay Clayton came after Washington scaled back staffing at its mission in Saudi Arabia earlier this week in response to intensified attacks by Iran-allied Houthi forces.
Those attacks on one front of the nearly eight-month-old regional conflict triggered by the Iran war have coincided with a rise in attacks on commercial shipping in the Strait of Hormuz.
Earlier this month, sources told Reuters Iran was preparing a more forceful response if the United States resumes large-scale military attacks on its territory.
"Specifically, through various channels, we have received intelligence that Iran has substantially increased efforts to target US personnel," Clayton wrote to US Secretary of State Marco Rubio in a message dated Oct 23.
"The lethal plotting efforts by the Iranians against US citizens have reached advanced stages in both the Middle East and certain other countries," he added.
CIA Director John Ratcliffe was also copied in on the unclassified memo which was first reported by Axios.
The State Department on Wednesday ordered non-emergency government employees and their family members to leave US diplomatic missions in Saudi Arabia and planned to withdraw family members from a number of other missions in the region, US officials told Reuters.
On Saturday, the US State Department updated travel advisories for more than a dozen countries across the Gulf and West Asia, including Saudi Arabia, Qatar, the United Arab Emirates, Iraq and Israel, citing an increased threat from Iran-aligned Houthis.
"When it comes to our staffing at embassies, we constantly evaluate new information about the security environment," a US official said.
"The threat of Houthi attacks and of Iranian terror plots targeting US officials are informing our decisions on staffing adjustments for embassies on RO Status in the region," the official said.
The Restricted Operations Status provides the State Department more flexibility in personnel decisions without having to publicly announce a change in the status of a mission.
The Iran war is a week away from entering its ninth month and has significantly weighed on the approval ratings of US President Donald Trump ahead of midterm elections on Nov 3.